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Thread: NiMh and solar charging
www.piclist.com/techref/power/batterys.htm?key=nimh
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face BY : Russell McMahon email (remove spam text)



> My idea at this stage is 600mAh NiMh 3.6V battery pack (as
> the circuit runs at 2.7-4v) and charge this via a 6V solar
> cell. (about 3-4days with no sun I hope..)
>
> I intend to use 2 fixed resistors as a trickle / fast charge
> cct, with the fast charge one switched in when battery falls
> below 3v, and switch to trickle when fully charged. (fast charge
> at about 60ma) also given that max charge will only be when
> the solar cell gets the good sunshine (plenty of that in Aus at
> the moment..)
>
> Does this sound a feasibly plan?
> Or is there a much better solution not using something complicated
> like NiMh batteries?

The following is MORE complicated than NiMH batteries :-) - but may be felt
to be worth the effort.

If solar cell optimisation is of interest (max run time for a given panel or
minimum panel for a given run time) then running a switching regulator
between panels and battery can very significantly improve your delivered
power. A solar panel has a very very wide swing in available power and
voltage output and if you want to make use of the available energy at either
end of the day or during partial cloud you need an "adaptable load". For
very best results you need an optimising controller * to hold the panel at
its current peak power point and this is not as trivial as it may seem, but
a "dumb" switcher is an excellent start.

Similarly, a switcher between battery and load may well do the same but
probably not as spectacularly due to the relatively flat voltage curve of
the NiMH cell.

At these power levels there are numerous small off the shelf switcher ICs
that allow a design with minimal effort and good probability of a result
better than just hooking it up.

* - a cell will deliver peak power at different voltages depending on the
level of insolation. Cell voltage depends on both insolation and load. If
you load the cell to the same voltage under all conditions the output will
not be optimal in most cases. Working out where the optimum point lies can
be achieved either by making small excursions either side of the current
load point and seeing which way the power output goes or by use of a
separate monitoring cell with a fixed load whose insolation/voltage
characteristics are predetermined. Ability to measure both current and
voltage is indicated as effective load resistance is, of course, dynamic.

Now all you have to do is work out an optimising controller for your
switcher(s) that draws far less than 5mA equivalent at Vout :-)



       RM

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